The Lower Thermosphere during the Northern Hemisphere Winter of 2009: a modeling study using High-Altitude Data Assimilation Products in WACCM-X

We present numerical simulations using the Whole Atmosphere Community Climate Model, extended version, constrained below 90 km by a combination of NASA's MERRA and the U.S. Navy's NOGAPS-ALPHA assimilation products. The period examined is January and February 2009, when a large stratospheric warming occurred on 24 January 2009, with anomalous circulation persisting for several weeks after the event. In this study, we focus on the dynamical response of the lower thermosphere up to 200 km. We find evidence of migrating and non-migrating tides, Rossby and Rossby-gravity modes, and Kelvin waves, whose amplitudes appear to be modulated at the times leading and following the stratospheric warming. While the Rossby, Rossby-gravity and Kelvin modes are rapidly dissipated in the lower thermosphere (above 110 km), the tides maintain substantial amplitude throughout the thermosphere, but their vertical structure becomes external above about 120-150 km. Most waves identified in the simulations decrease in amplitude in the thermosphere, indicating remote forcing from below and strong dissipation by molecular diffusion at high altitudes; however, the amplitude of the migrating DW1 tide increases in the thermosphere suggesting in situ forcing. We show that the amplitude of the tides (such as the DW1) changes as the background wind alters the vorticity in the tropics, which broadens or narrows the tropical waveguide. Our results also suggest that fast Rossby normal modes (periods ≤ 10 days) are excited by instability of the zonal mean wind distribution following the stratospheric warming.